A moving image shot by an imaging device such as a digital video camera or a digital still camera, includes noise attributable to the shooting environment or characteristics of an image sensor. Examples of noise include photon shot noise, dark noise, fixed pattern noise, and circuit noise or the like. Because such noise deteriorates the quality of an image (image quality), noise reduction (NR) is performed to remove (or reduce) the noise in the moving image.
A well-known noise reduction method is a method of smoothing a pixel value of a target pixel, which is a processing target, using pixel values of neighboring pixels of the target pixel. For smoothing a pixel value, for example, used is a method of obtaining an average of the values of the target pixel and the neighboring pixels, or a method of obtaining a median (median filter) of the values of the target pixel and the neighboring pixels. In these methods, noise reduction is performed in a spatial direction using pixel values within one image frame, and thus is called spatial noise reduction.
For a moving image, known is a method of smoothing a pixel value of a target pixel, using correlations among a plurality of images that are successive in a temporal direction. For smoothing a pixel value, for example, used is a method of obtaining a weighted average of the pixel value of the target pixel and a pixel value at the same coordinates in a temporally preceding image. In this method, noise reduction is performed in the temporal direction using pixel values in a plurality of image frames, and thus is called temporal noise reduction.
In addition, as a combinational method of the spatial noise reduction and the temporal noise reduction, there exists a technology which, based on a motion detection result of a moving image, selects the temporal noise reduction when a target pixel is in a motionless image area and selects the spatial noise reduction when the target pixel is in a motion image area (for example, see Patent Literature 1).